1. Field of the Invention
The present invention relates to a method for deriving spectral sensitivity characteristics suitably used for obtaining the spectral sensitivity characteristic in a color image inputting device, in particular, in a color scanner, and to an apparatus for the same.
2. Description of the Related Art
It is becoming important to accurately reproduce colors of an original subject from color image signals taken from a color image signal input device such as color scanners, digital cameras or a like achieving remarkably widespread use in recent years by using a color image signal output device such as color monitors, color printers or a like.
In response to such needs, a method of image processing is disclosed (for example, in Japanese Patent Application No. Hei 7-20585) in which a color adjustment is made to colors produced from an image signal inputted from the color image signal input device. In the disclosed image processing, the image signal inputted from the color image signal input device is transformed to image data (X, Y and Z values) in a CIE (Commisson Internationale de L'eclarge) XYZ colorimetric system and, after the transformed image data has been transformed to data (L*, a* and b* values) in a uniform color space (for example, CIE LAB color space) based on a reference white, colors desired to be amended are designated and the designated colors are again transformed to the image data (X′, Y′ and Z′ values) in the CIE XYZ colorimetric system by increasing and decreasing the L*, a*, b* values according a difference between the color space data possessed by the designated color and color space data obtained after the transforming processing. In the image processing, since an increased and decreased amount of each of coordinate component values making up color space data is calibrated while seeing the input image by a monitor, it is possible to make the amendment to the input image so as to have a desired color balance.
Thus, in the conventional technology in which the image processing is performed by using the X, Y and Z values in the CIE XYZ system or the L*, a*, b* in the CIE LAB color space, color matching functions in the CIE XYZ colorimetric system are used as standard color matching functions. However, it is generally impossible to make equal the X, Y and Z values of two stimuli each having a different spectral reflectance factor perceived to be the same in color in a specific illuminant, by observation in a specific color matching evaluation experiment, by using the color matching functions in the CIE XYZ colorimetric system. Due to this, two stimuli perceived by the observation to be same in color in the specific illuminant are considered to be different in color and, as a result, after the input image signal is acquired, the color calibration is performed even on the color stimuli requiring no amendment.
That is, in the conventional image processing, to transform color data in a colorimetric system taken from the color image input device to that in other colorimetric system, it is necessary to obtain spectral sensitivity characteristics of the input device. The spectral sensitivity characteristics are represented by tristimulus values and each stimulus value is calculated by using a standard color matching function designated by the CIE. However, when the standard color matching function designated by the CIE is used, even a color perceived by a human to be the same in color is considered to be different in color and therefore it is impossible to obtain proper spectral sensitivity characteristics by the conventional method of deriving the spectral sensitivity characteristic and is difficult to achieve a proper transformation of color data by the conventional method.